It is known from G. E. Bodeep et al.: "Semiconductor Lasers Versus External Modulators: A Comparison of Nonlinear Distortion for Lightwave Subcarrier CATV Applications," IEEE Photonics Technology Letters, Vol. 1, No. 11, November 1989, pages 401 through 403, that the laser of this type of transmitter can be amplitude-modulated by two different modulating methods, both of which are marked by drawbacks.
In the first method of modulation, a direct modulation, the laser is electrically modulated with the intelligence signal. Here, an undesired frequency modulation occurs in addition to the desired amplitude modulation. This effect is generally referred to as frequency chirp. The frequency chirp broadens the intrinsic line width of the laser, creating distortions in the transmitted signal caused by fiber dispersion within only a few kilometers of a transmission route.
In the second method of modulation, an indirect modulation, the carrier emitted from the laser is optically modulated with the intelligence signal by means of an intensity modulator which follows the laser. The laser emits at a constant power, preventing frequency chirp. Nonetheless, the maximum transmittable power of a transmitter operating according to this principle is limited by Brillouin scattering in the fibers of the transmission route connected to the optical transmitter. The effect of Brillouin scattering is recognizable at as low as approximately 10 mW of transmitted power. This effect of Brillouin scattering increases with the spectral power density and the length of the optical fibers of the transmission route and leads likewise to a rise in distortions and noise of the transmitted signal.
It was shown in D. Cotter et at.: "Suppression of Stimulated Brillouin Scattering During Transmission of High-Powered Narrowband Laser Light in Monomode Fibre," Electronics Letters, 22nd Jul. 1982, Vol. 18, No. 15, pages 638 through 640, that Brillouin scattering is sharply reduced by means of a constant phase or frequency alteration of an optically amplitude-modulated signal and completely suppressed in an ideal case. It is indicated for this purpose that an additional optical phase modulator be inserted in the optical path of the transmitter or an additional laser be used. In the experiments upon which the aforementioned source is based, a solid laser is employed which can emit in two modes that are 270 MHz apart. This is, however, meaningful only in the dispersion minimum of the fibers.